I don't think that changes in weight will affect frequency as that is a function of motor/drive speed. But I do think that more mass means more inertia. For a given amount of energy being supplied to the "tub" by the drive, as mass (and inertia) increases the less the tub will move with each cycle with the corresponding reduction in cleaning peformance.

So....as the mass (inertia) increases you have to supply more energy and you do that with more offset weight.